1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, which prevents, in an internal combustion engine performing a fuel cut control at the time of deceleration of a vehicle or the like, an exhaust gas from deteriorating after a return from the fuel cut.
2. Description of the Related Art
A three-way catalyst used for purifying an exhaust gas of an internal combustion engine oxidizes or reduces hydrocarbon, carbon monoxide, and nitrogen oxide as harmful components in the exhaust gas under stoichiometric atmosphere and converts them into harmless components, and further has such properties as to store oxygen when the oxygen used at the time of oxidation or reduction is excessive and as to release the stored oxygen when it is insufficient. When the internal combustion engine carries out fuel cut, since fuel is not supplied, and combustion in a cylinder is not performed, a large amount of oxygen is introduced into an exhaust system of the internal combustion engine, and the oxygen is stored in the three-way catalyst provided in the exhaust system. Thus, after a return from the fuel cut control, a difference occurs between the oxygen concentration at the upstream side of the three-way catalyst and the oxygen concentration in the actual three-way catalyst.
In the state as stated above, even if feedback control of an air-fuel ratio is performed to purify the exhaust gas, oxygen becomes excessive by the oxygen stored in the three-way catalyst, the air-fuel ratio can not be suitably controlled until the stored oxygen is consumed, and the oxidation/reduction reaction by the three-way catalyst can not be sufficiently used. Accordingly, until the stored oxygen is consumed, especially the amount of reaction to nitrogen oxide is lowered, and the nitrogen oxide in the exhaust gas is increased. In order to prevent this, it becomes necessary to change, when the fuel cut state is returned to a fuel supply state, the content of the air-fuel ratio control from that at the normal time.
As a technique for controlling the air-fuel ratio to suppress the increase of nitrogen oxide as stated above, there is a technique disclosed in, for example, patent document 1 (JP-A-05-26076 (pages 3 to 4, FIGS. 1 and 4)). The technique disclosed in this document is such that a main oxygen sensor is provided at the upstream side of a three-way catalyst, a sub-oxygen sensor is provided at the downstream side thereof, a feedback correction coefficient is increased or decreased according to a detection signal of oxygen concentration detected by the main oxygen sensor to feedback control an air-fuel ratio of an air-fuel mixture, a control constant used for the increase/decrease calculation of the feedback correction coefficient is changed according to a signal from the sub-oxygen sensor, and a deviation from the control center of the feedback control is corrected, in which the control constant is offset to a rich side during a period from the time point when the fuel cut is released to the time point when a signal of the sub-oxygen sensor is changed to a rich state, so that the consumption of oxygen stored in the three-way catalyst is accelerated, and the period when a purification rate of nitrogen oxide is deteriorated is shortened.
However, as in the technique disclosed in the prior art document, when the control constant is continuously offset to the rich side during a period until when the signal of the sub-oxygen sensor provided at the downstream side of the three-way catalyst is changed to the rich state after the fuel cut is released, although the air-fuel ratio in the three-way catalyst is made suitable and the nitrogen oxide can be purified, there occurs a state of excessive fuel supply, and carbon monoxide in the exhaust gas is increased. This state will be described below with reference to FIG. 4 of patent document 1.
When the fuel cut control is started, the output of the sub-oxygen sensor is shifted to the lean side, and lean output is continued. Since combustion is not performed during the period of the fuel cut, oxygen continues to be stored in the three-way catalyst, and is finally stored up to the limit of storage capacity. When the fuel cut is released, the control constant of the feedback is offset to the rich side during the period until when the signal of the sub-oxygen sensor is changed to the rich side, and the oxygen stored in the three-way catalyst is consumed, so that the output signal of the sub-oxygen sensor is changed from the lean side to the rich side. To this point, the description as shown in FIG. 4 of patent document 1 is appropriate, however, even if the control constant of the feedback is returned to a normal value in response to the shift of the signal of the sub-oxygen sensor to the rich side, the rich state of the air-fuel ratio continues for a specified time according to a time delay of the control, and since the stored oxygen of the three-way catalyst is wholly consumed, carbon monoxide or the like can not be reacted, and the carbon monoxide or the like in the exhaust gas is increased.